It is our intention to construct a cryogenic absorbed dose calorimeter with a sensitivity approximately an order of magnitude higher than any presently employed calorimeter. This device will be used to measure several quantities of importance to radiation dosimetry such as W for TE gas, stopping power ratios between ion chamber wall and gas and kerma in extended media of biologically important constituent elements (H,C,O,N) irradiated by neutrons and heavy charged particles, as well as high energy photons and electrons. Measurements will also be carried out to determine W for monoenergetic neutrons and broad cyclotron-produced neutron spectra. Complementary measurements will be made using ionization chambers constructed to be identical in size and shape to the calorimeter absorber. The specific ionization to dose conversion factors will be compared with present cavity ionization theories in the radiation fields mentioned above. This development should also establish the absorbed dose calorimeter as the preferred standardizing laboratory method for fundamental calibrations, and thus free the standard laboratories from dependence upon the more indirect method based on cavity ionization and its associated conversion factors. In addition, this research will provide mesaurements of quantities which will improve the accuracy of routine ionization measurements employed in neutron and particle, as well as conventional radiotherapy.